So, I purchased a boat a month ago and the prior owner has a 16 inch 3 bladed prop installed; unknown pitch. The boat has a Yanmar 4JHE and 2:17 gearing. The prior owner tells me the boat cruised well at 1800RPM, and it does, getting about 7kts, no black smoke. Max RPM while cruising is 2200RPM.

Now, I read that you should be cruising around 80% of your max rated RPM, in this case 3600. So, my questions is; am I doing any damage by cruising at 1800 RPM? What specifically?

Sounds like you are severely overpropped. Your engine is doing the work it was designed to do at 80% of it's rated HP (push the boat at hull speed) at 50% of WOT. That means the engine is working almost at it's maximum (400 rpm below maximum rpm) to move the boat but only has half or less of it's horsepower to work with. Get the prop re-pitched, or a new prop, your engine will thank you (it will last longer and be less prone to blow a head gasket, etc). Call a reputable prop shop and ask them, they'll undoubtedly have horror stories for you.

The way I understand it there are two separate items. Your prop should be pitched allow you to get up to 90% of the max engine power (SHP). You can find that RPM in a curve provided by the manufacturer but using 90% of max rpm I think works as well. Cruising speed should be at from 70 to 80% power. The reason for this is that the diesel wants to be at that temperature to fully burn it's fuel. Operating at lower powers results in carbon build-up and fouling of the cylinders and valves. The most common diesel problem with sailboat auxiliaries is not operating long enough at power to get to the proper temperatures. Typically they're started up to get underway then shut down before getting up to temperature with the reverse on return. This kills diesels.

..Now, I read that you should be cruising around 80% of your max rated RPM, in this case 3600. So, my questions is; am I doing any damage by cruising at 1800 RPM? What specifically?

I'm not familiar with the specs of your motor, but suspect you mean that the max published rpm is 3600.

You can not check this at the dock in neutral. You should reach the max rpm with the throttle wide open and underway. Tachometers are notoriously inaccurate, but you should be within 100rpm or so. If you aren't, you are either over or under pitched.

Think of the pitch of your prop like the gears of a manual transmission in a car. If underpitched, its like running down the highway in 1st gear. If overpitched, its like going up hill in 5th gear. Both are bad for your motor.

That's based on a Hull speed should be 8 knts, not 7 (LWL = 35).
3240 rpm (90% of max) / 2.17 reduction x 12 (pitch, inches) with comes out to 1493 feet per minute.
Once you figure the slippage (per David Gerr's chart) at 48% you get 776 feet per minute - just under 8kts (810 feet per minute).

It's important to note that Gerr's slip chart is an ESTIMATE - so you get some variation in numbers.

Okay, that's what you should have had, now you have a 3 blade of unknown pitch
You can get a pretty good estimate on your pitch by reverse engineering the speed/rpm:

Since you are saying you are going at 1800 rpm, that means you have a shaft rotation of 829 RPM and are moving 708 feet per minute (7 kts).
That comes out to a pitch of almost 20 (829 * 20 (pitch) *.52 (slippage) / 12 (convert feet to inch) = 718 feet per minute

That's a bit much in pitch and explains why you can't get 90% of max RPM.

I'm running the same engine on my Irwin 38 - with a 16 inch 3 blade MaxProp, pitch set to 12.

My boat is also overpropped. I was going to change back to the two blade that came stored onboard. I did wonder why the original owner changed it though. I learned why, motoring into a steep chop. She wants the power there more than I need the speed in smooth water. Your new three blade may be the right compromise.
I know the books all talk about the dangers of low rpm, etc. But my engine has run fine for decades like that.

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